There are a wide range of systems for establishing and maintaining short range wireless communication links. Many of these systems require a line of sight link, do not support two way communication, do not provide low power consumption characteristics, or flexible connection topologies. Two examples of short range wireless communication protocols are Bluetooth and low-end radio.
As the Bluetooth specification matures, there are a greater number of wireless devices implementing Bluetooth as a standard feature. However, the power consumption requirements and implementation costs make it prohibitive to implement in a host of wireless devices that have low power capacity. Nokia Research Center has developed a radio technology, called low-end radio, which is based on the Bluetooth Specification and may utilize at least the analog parts of the Bluetooth radio, to provide wireless communication links between devices that have limited power resources. The low-end radio protocol enables significant power and cost reductions over Bluetooth wireless devices and accommodates devices having limited power resources.
Low-end radio is discussed in International Publication Number WO 02/073893. The low-end radio protocol enables devices with low power consumption requirements to communicate wirelessly over a certain frequency band. Unlike Bluetooth, low-end radio does not implement a frequency hopping routine or a transmission slot system. This results in a simpler, less complex system than a standard Bluetooth implementation. The low-end radio protocol divides the communication frequency band into a multitude of communication channels.
A device with low-end radio protocol provides more efficient power savings when compared with the standard Bluetooth devices. However, additional power savings would enable low-end radio applications to become even more diverse and widespread. One possible method for reducing power consumption involves implementing low-end radio with Carrier Sensing Multiple Access and collision avoidance.
U.S. patent application Ser. No. 10/224,768, entitled “Carrier Sensing Multiple Access with Collision Avoidance Scheme Optimized For A Priori Known Carrier Usage For Low Duty Systems,” (CSMA with collision avoidance) describes systems and methods for a communication system implementing a short range wireless communication link between user devices. The communication system provides a low power solution utilizing an optimized combination of carrier sensing and frequency division multiple access to avoid collisions. The optimization described in that patent application implements a random mean zero value offset appended to a device's transmission frames, when transmissions are sensed on a transmission channel. The offset increases the probability that a first advertising device will recognize another device's transmissions (i.e., determine that a channel is busy) and avoid simultaneous transmissions that result in transmission collisions. Upon recognizing another device's transmissions, the first device may shift its transmission frame to avoid transmission collisions between devices. Consequently, through efficient management of device transmissions, fewer retransmissions are necessary and power consumption is reduced. The carrier sensing communication links may be implemented in either a StandAlone LowRate system, or in a device with a pre-existing Bluetooth implementation.
The Bluetooth specification also has a low-power operating mode that attempts to minimize power consumption. A Bluetooth piconet includes a master and anywhere from one to seven active slaves communicating. Bluetooth implements a frequency hopping system derived from the master's Bluetooth clock signal and the device address. Generally, the hop rate in a normal connection is 1600 hops/s. Transmissions are conducted during specified time slots that are determined according to a predetermined hopping scheme, (e.g., the duration of a time slot is 625 μs). According to the Bluetooth protocol, a Master device may start transmitting only in even-numbered slots, whereas the slave devices may transmit in odd-numbered slots. The data packets may occupy 1, 3 or 5 slots. The whole packet is always transmitted in the same channel. The master polls one slave at the time. Each slave transmits a response message back to the master after receiving the poll. The active slave devices recognize their packets by processing a 3-bit active member addresses in the packet header. Further interaction between a master and a slave depends upon which of three types of master/slave communication links is established.
There are three different communication link types that a master and active slaves in the Bluetooth low power mode may establish: Synchronous Connection-Oriented (SCO) link, Extended Synchronous Connection-Oriented (eSCO) link, and Asynchronous Connection-Less (ACL) link. Synchronous links establish point-to-point links between a master and a single slave in the piconet. A master can manage up to three SCO links by using reserved slots at regular intervals. In SCO links, packets are never retransmitted, whereas eSCO links may have an additional retransmission window after the reserved transmission slots. An ACL link may be a point-to-multi-point link between a master and all of the slaves participating on the piconet. A master can establish an ACL link on a per-slot basis to any slave, in transmission slots not reserved for the synchronous links.
The Bluetooth ACL link implements a system where slaves may enter a sleep state for a predetermined length of time. For example, the Bluetooth protocol implements a low power mode (sniff mode) for slaves which participate on ACL links. Sniff mode reduces the number of the time slots in which the master can start transmission to a specific slave. The master can start transmission only in specified time slots, called sniff slots, which are spaced regularly within a time interval (Tsniff). The slave in sniff mode starts listening for sniff slots after a predetermined delay (Dsniff).
Fifteen packet types are defined for data links, for different link types, data, error handling and length. Additionally, there are 5 common packets for control purposes and connection establishment. Each synchronous (voice) channel supports a 64 kb/s data rate in each direction, whereas the asynchronous channel can support a maximum data rate of 723.2 kb/s asymmetric (and still up to 57.6 kb/s in the return direction), or 433.9 kb/s symmetric.
Despite the improved power consumption characteristics associated with implementing low-end radio and Bluetooth's low power mode in coordination with CSMA with collision avoidance, these protocols do not satisfy the power requirements of a multitude of wireless devices and applications with low power requirements.